An electro-mechanical pressure sensor generally comprises a membrane made of silicon or silicon alloy on the front face of which piezoelectric strain gage mounted as a Wheatstone bridge and connected to an electronic processing unit by connection wires are added. The back face, opposite the one supporting the gages, is exposed to a pressure to be measured, which, when causing the membrane to buckle, actuates the gages and enables making an electric measuring of pressure. The membrane is generally mounted on a substrate which is made of silicon too. As silicon is particularly sensitive to electrochemical attacks, the membrane is mounted at the end of a conduit filled with a transfer fluid, generally silicone oil. The other end of the conduit is closed by a stainless steel pellet, the outer face of which is in contact with the fluid, the pressure of which has to be measured. The pressure applied to the stainless steel pellet is transmitted, through the transfer fluid, to the silicon membrane and measured by the processing unit from the signals supplied by the strain gages. The electric signal generated by the processing unit is then transmitted to a communication network.
The thus obtained sensor is generally cumbersome, heavy and expensive, specifically because of the presence of the oil-filled conduit and the sealing elements associated thereto. As a matter of fact, the oil has to be absolutely incompressible and such oils are expensive and freeze at a low temperature, and thus transmit the vibrations. When they are not totally free of impurities and/or free radicals, such oils generate electric drifts when they are exposed to electric voltage. The cylindrical conduit must be filled with extreme precautions since the presence of air in the conduit would make the sensor inaccurate or even inoperative. Such operation and the checking thereof increase the cost of production of the sensor. Eventually, such sensor is extremely sensitive to quick variations in the temperature of the fluid, the pressure of which has to be measured. As a matter of fact, although piezoelectric sensors are well known as having a reduced sensitivity to temperature variations, the behaviour of the transfer fluid and the conduit entail mistakes which can be corrected with difficulty. Finally, at extremely low temperatures, the transfer fluid may freeze and make the sensor inoperative.